Your phone cannot be used as a USB by itself. Both the phone and the computer must have an OS running on them already to utilize the MTP protocol, which allows you to transfer files between them.
Edit: A possible approach:
Warning, this is not secure and kind of defeats the point of Tails!
In this scenario, the only way to boot Tails from a phone would be to first boot a different OS on the computer, plug the phone in and mount it using the MTP protocol, then boot a virtual machine image stored on the phone with QEMU or similar.
Your phone cannot be used as a USB by itself. Both the phone and the computer must have an OS running on them already to utilize the MTP protocol, which allows you to transfer files between them.
Edit: A possible approach:
Warning, this is not secure and kind of defeats the point of Tails!
In this scenario, the only way to boot Tails from a phone would be to first boot a different OS on the computer, plug the phone in and mount it using the MTP protocol, then boot a virtual machine image stored on the phone with QEMU or similar.
It’s a shame that it needs to be a smart decide, and can’t just act as a dumb device with an iso.
Perhaps a future build of android will allow it/emulate it/spoof it for this purpose.